Dynamic optical devices

ABSTRACT

The invention provides an optical device, including a light-transmissive substrate, and a pair of different, parallel gratings including a first grating and second grating, located on the substrate at a constant distance from each other, each of the pair of parallel gratings including at least one sequence of a plurality of parallel lines, wherein the spacings between the lines gradually increase from one edge of the grating up to a maximum distance between the lines, and wherein the arrangement of lines in the second grating is in the same direction as that of the first grating. A system utilizing a plurality of such optical devices is also disclosed.

This application is a division of application Ser. No. 10/311,564 filedon Jun. 10, 2003 for Dynamic Optical Devices.

FIELD OF THE INVENTION

The present invention relates to diffractive optical devices (DOEs), andto devices which include a plurality of chirped diffractive opticalelements carried by a common light-transmissive substrate.

The present invention is capable of being implemented in a large memberof applications. For purposes of example only, implementations indivision multiplexing/demultiplexing systems, compact optical switchesand compact optical scanners are indicated herein.

BACKGROUND OF THE INVENTION

Recently, there have been significant advances in optical fibertechnology for telecommunication systems. One of the proposed methods ofexploiting the high potential bandwidth of optical fibers moreefficiently, is by wavelength division multiplexing (WDM). With thistechnique, a large number of communication channels can be transmittedsimultaneously over a single fiber. Various systems for implementing WMhave been proposed, including systems based on birefringent materials,surface relief gratings, Mach-Zender interferometry and waveguides.These proposed systems generally suffer from low efficiencies, or from astrict limitation on the number of possible channels.

Another proposed approach is to use a thick reflection hologram.However, the necessity to use a conventional aspheric lens forcollimating and/or focusing the light waves makes such systems bulky andspace consuming. Furthermore, a single holographic element is verysensitive to the signal wavelength, which is usually strongly dependenton temperature.

In many optical systems, scanning of a plane wave over a wide field ofview, or linear scanning of a focused beam on a plane, is required. Afew examples are angular scanners for Laser-Radar, whereby thetransmitted narrow beam is to cover a solid angle much wider than theangular divergence of the beam; aiming systems in which the centralaiming point moves as a function of the target range and velocity;linear scanners for laser printers or plotters, and others. In theexisting systems, beam steering is performed with conventional opticalelements, such as a polygonal mirror or a pair of prisms. These systemssuffer from various drawbacks: the scanning unit is relatively large andheavy, limiting the performance of systems in which compactness is arequirement; mass production is quite expensive; the scanning rate isseverely limited by the mechanical system; rotating systems usuallysuffer from wobble which must be restrained in order to allow accuratescanning.

Several proposals have been made to perform beam steering by microlensarray translation with either diffractive or refractive lenses. Theseapproaches usually suffer from high aberrations at small f-numbers. Inaddition, they must rely on fairly complicated and costly equipment,which often limits the performance of the microlens arrays.

DISCLOSURE OF THE INVENTION

It is therefore a broad object of the present invention to provide acompact, relatively inexpensive, accurate and simple beam steeringoptical device having a high scanning rate.

It is a further object of the invention to provide a compact, relativelyinexpensive, accurate and simple optical device for wavelength divisionmultiplexion/demultiplexion having high spectral separation.

It is a still further object of the invention to provide an opticaldevice having a substrate wherein a slight change in the refractiveindex of the substrate will cause an angular deviation in the outputbeam.

It is a still further object of the invention to provide an opticaldevice providing a large deviation coefficient, so that with a minuterefractive index change, significant deviation in the output beam isachieved.

In accordance with the present invention, there is therefore provided anoptical device, comprising a light-transmissive substrate, and a pair ofdifferent, parallel gratings including a first grating and a secondgrating, located on said substrate at a constant distance from eachother, each of said pair of parallel gratings comprising at least onesequence of a plurality of parallel lines, wherein the spacings betweensaid lines gradually increase from one edge of the grating up to amaximum distance between said lines, and wherein the arrangement oflines in said second grating is in the same direction as that of saidfirst grating.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in connection with certain preferredembodiments with reference to the following illustrative figures so thatit may be more fully understood.

With specific reference now to the figures in detail, it is stressedthat the particulars shown are by way of example and for purposes ofillustrative discussion of the preferred embodiments of the presentinvention only, and are presented in the cause of providing what isbelieved to be the most useful and readily understood description of theprinciples and conceptual aspects of the invention. In this regard, noattempt is made to show structural details of the invention in moredetail than is necessary for a fundamental understanding of theinvention, the description taken with the drawings making apparent tothose skilled in the art how the several forms of the invention may beembodied in practice.

In the drawings:

FIGS. 1 a, 1 b, 1 c, 1 d, 1 e, 1 f, 1 g and 1 h illustrate the geometryof some possible embodiments of a device according to the presentinvention;

FIGS. 2 a and 2 b schematically illustrate ray tracing output whenpassing through two gratings of the device according to the invention;

FIG. 3 schematically illustrates an iterative procedure according to theinvention;

FIG. 4 schematically illustrates the conversion of a plane wave to alinear point scanner by means of a focusing lens;

FIG. 5 schematically illustrates the utilization of the double-gratingconfiguration, constructed to provide a wavelength divisiondemultiplexing system;

FIGS. 6 a and 6 b schematically illustrate the utilization of thedouble-grating configuration, constructed to be employed for lightintensity attenuation or in light amplitude modulation;

FIG. 7 schematically illustrates an array of identical grating couples;

FIG. 8 schematically illustrates a further embodiment for reducing thethickness of the substrate in order to achieve a more compact device;

FIG. 9 is a graph illustrating the results of simulations whichcalculate the dispersion of a system as a function of the refractiveindex of the substrate, incorporating the device of the presentinvention;

FIG. 10 is a graph illustrating the results of simulations whichcalculate the output angle from the second grating as a function of therefractive index of the substrate, incorporating the device of thepresent invention;

FIG. 11 is a graph illustrating the results of simulations whichcalculate the wavelength, at an output angle of ρ=20° as a function ofthe refractive index of the substrate of the device according to thepresent invention;

FIGS. 12 a and 12 b are graphs illustrating the results of simulationswhich calculate the grating period (in line-pairs/mm) of gratings G₁ andG₂ as a function of x (FIG. 12 a) and ξ (FIG. 12 b), respectively;

FIG. 13 is a side view of a first stage of a switching systemincorporating a device according to the present invention;

FIG. 14 is a side view of a second stage of the switching system of FIG.13;

FIG. 15 is a top view of an optical switching system;

FIG. 16 is a schematic diagram illustrating S-polarization of anincoming beam, and

FIG. 17 is a schematic diagram illustrating S-polarization of an uniformand symmetrical incoming beam.

DETAILED DESCRIPTION

In its simplest form, as shown in FIG. 1 a, the optical device 2 of thepresent invention includes a light-transmissive substrate 4 having twofacets or surfaces 6, 8. A plurality of parallel lines 10 are made onsurface 6, constituting a first grating A. The spacings between thelines increase from one edge 12 of the surface to its other edge 14,according to mathematical formulae. The arrangement of lines 16 onsurface 8 forms a second grating B. The spacings between the parallellines 16 of second grating B increase in the same direction as those ofgrating A.

According to the embodiment of FIG. 1 b, the surfaces 6 and 8,respectively, bear gratings C and D, each grating being formed ofparallel lines, the spacings of which increase from one edge 18 and 20,respectively, of the surfaces to their centers, and then decreasetowards the other respective edge 22, 24, in a symmetrical manner.

FIG. 1 c depicts a modification of the arrangement of FIG. 1 b, whereinthe substrate 4 bears two gratings A, A′ and B, B′ according to thearrangement shown in FIG. 1 a. Similarly, the substrate of FIG. 1 dbears gratings C, C′ and D, D′ on its surfaces 6 and 8, as in thearrangement shown in FIG. 1 b.

The embodiment of FIG. 1 e includes a substrate 4 having gratings A andB as shown in FIG. 1 a, formed on a single surface 8. Similarly, FIG. 1f illustrates gratings C and D as in FIG. 1 b, formed on surface 8; FIG.1 g illustrates the gratings A, A′, B, B′ of FIG. 1 c, formed on asingle surface 8, and FIG. 1 h shows gratings C, C′, D, D′ formed on asingle surface 8.

In the double grating system shown in FIGS. 2 a and 2 b, a monochromaticplane wave W is coupled inside a light-transmissive substrate 4 by afirst grating G₁ on surface 8 and then is coupled out by the secondgrating G₂ formed on surface 6. The refractive index of the substratecan be dynamically controlled by external means, including, but notlimited to, applying an electric field to the substrate or byillumination with a strong short-wavelength light source (not shown).There are many materials with which the electro-optic effect can be usedto control the refractive index of the material. One such well-knownmaterial is Lithium-Niobate (LiNbO₃), which is commercially availableand which has a very fast time response, in the order of 10⁻⁹ second.However, many other materials, crystals and polymers, can just as wellbe used for the desired purpose.

The present invention is intended to provide an optical system wherein achange in the refractive index of the substrate yields an angulardeviation of the output wave. That is, when the refractive index is ν₁,the output wave W_(o) emerges from the second grating G₂ formed onsurface 6, at an angle ρ₁ with respect to the substrate plane (FIG. 2a). However, when the refractive index is changed to ν₂, the output waveW_(o) is deviated by an angle Δρ, that is, the output wave emerges fromgrating G₂ at a different angle ρ₂ to the substrate plane (FIG. 2 b).Hence, a continuous change in the refractive index induces a continuousangular steering of the output wave. This angular steering can beconverted into linear scanning of a focused beam by means of anappropriate converging lens.

It is assumed that the input wave W_(i) which comprises collimated lightwaves 101,102 impinges on the first grating G₁ at an angle β_(o) to thenormal of the substrate. The input and output rays remain in the samemeridional plane without any loss of generality. Therefore, the twograting functions are invariant in y (the axis normal to the meridionalplane), and depend only on x, the plane of grating G₁. The distance dbetween G₁ and G₂ is normalized to 1. In the iterative procedure, shownin FIG. 3, an initial point x_(o) is chosen on grating G₁. The incomingray W_(i) of λ₁ is traced (solid line) from x_(o) to a point ξ₀ ongrating G2 in a chosen direction β₁(x_(o)). The grating function of G₁at x_(o) is

$\begin{matrix}{{{\phi_{1}\left( x_{0} \right)} = {\frac{2\pi}{\lambda_{1}}\left\lbrack {{v_{1}\sin\;{\beta_{1}\left( x_{o} \right)}} + {\sin\;\beta_{0}}} \right\rbrack}},} & (1)\end{matrix}$where both β₀ and β₁(x₀) are defined to be positive in FIG. 3. Withoutany loss of generality, it is assumed that the output wave for therefractive index ν=ν₁ emerges at an angle ρ₁ to the substrate plane.Hence, the grating function of G₂ on ξ₀ is

$\begin{matrix}{{\phi_{2}\left( \xi_{0} \right)} = {\frac{2\pi}{\lambda_{1}}{\left( {{v_{1}\sin\;{\beta_{1}\left( x_{o} \right)}} + {\sin\;\rho_{1}}} \right).}}} & (2)\end{matrix}$

The refractive index form is now changed from ν₁ to ν₂ and, as a resultof the change in the refractive index, the output wave emerges from thesubstrate at an angle ρ₂ to the substrate plane. The direction of theimpinging ray at ξ₀ is

$\begin{matrix}{{v_{2}\sin\;{\beta_{1}^{o}\left( \xi_{0} \right)}} = {{{\frac{\lambda_{1}}{2\pi}{\phi_{2}\left( \xi_{0} \right)}} - {\sin\;\rho_{2}}} = {{v_{1}\sin\;{\beta_{1}\left( x_{0} \right)}} - {\sin\;\rho_{2}} + {\sin\;{\rho_{1}.}}}}} & (3)\end{matrix}$

Tracing a ray 26 into G₁ (dashed lines) calculates the grating functionφ₁ on x₁ as

$\begin{matrix}\begin{matrix}{{\phi_{1}\left( x_{1} \right)} = {\frac{2\pi}{\lambda_{1}}\left\lbrack {{v_{2}\sin\;{\beta_{1}^{o}\left( \xi_{o} \right)}} + {\sin\;\beta_{0}}} \right\rbrack}} \\{{= {\frac{2\pi}{\lambda_{1}}\left\lbrack {{v_{1}\sin\;{\beta_{1}\left( x_{o} \right)}} - {\delta\rho} + {\sin\;\beta_{0}}} \right\rbrack}},}\end{matrix} & (4)\end{matrix}$where the constant δρ is defined as δρ≡ sin ρ₂−sin ρ₁. By switching backto ν₁, the procedure can continue and the input angle at x₁ iscalculated to be

$\begin{matrix}{{{v_{1}\sin\;{\beta_{1}\left( x_{1} \right)}} = {{{\frac{\lambda_{1}}{2\pi}{\phi_{1}\left( x_{1} \right)}} - {\sin\;\beta_{0}}} = {{v_{1}\sin\;{\beta_{1}\left( x_{0} \right)}} - {\delta\rho}}}},} & (5)\end{matrix}$which yieldssin β₁(x ₁)−sin β₁(x ₀)=δρ/ν₁   (6)

Now, the iterative procedure continues to calculate φ₁ and φ₂ at thepoints x₁,x₂,x₃ x_(i) and ξ₁,ξ₂,ξ₃ . . . ξ_(i) respectively. Followingthe same procedure as in Equations 1-6, it can be found that for eachx_(i)sin β₁(x _(i+1))−sin β₁(x ₁)=δρ/ν₁   (7)

With this iterative procedure, the desired grating functions φ₁ and φ₂can be calculated only for a finite number of pairs. Knowing thediscrete values of these functions, the values of the other points onthe gratings can be calculated numerically by using interpolationmethods, however, since the number of known values is comparativelysmall, the interpolation procedure may be complicated andtime-consuming. Instead, analytic functions are presented herein, whichare relatively easy to compute for an iterative procedure.

As shown in FIG. 3, the distance Δx_(i)≡x_(i+1)−x₁ that the tracingprocedure “moves” each step along the x axis is approximately

$\begin{matrix}{{{\Delta\; x_{i}} \approx \frac{\Delta\;\beta_{i}}{\cos^{2}{\beta_{1}\left( x_{i} \right)}}},} & (8)\end{matrix}$where Δβ_(i) is given by

$\begin{matrix}{{{\Delta\;\beta_{i}} \approx \frac{{\sin\;{\beta_{1}\left( x_{i} \right)}} - {\sin\;{\beta_{1}^{o}\left( \xi_{i} \right)}}}{\cos\;{\beta_{1}\left( x_{i} \right)}}} = {\frac{{\left( {\begin{matrix}v_{1} \\v_{2}\end{matrix} - 1} \right)\sin\;{\beta_{1}\left( x_{i} \right)}} + \begin{matrix}{\delta\rho} \\v_{2}\end{matrix}}{\cos\;{\beta_{1}\left( x_{i} \right)}}.}} & (9)\end{matrix}$

Inserting Equation 9 into Equation 8 yields

$\begin{matrix}{{\Delta\; x_{i}} \approx {\frac{{\left( {\begin{matrix}v_{1} \\v_{2}\end{matrix} - 1} \right)\sin\;{\beta_{1}\left( x_{i} \right)}} + {\frac{v_{1}}{v_{2}}{\delta\rho}}}{\cos^{3}{\beta_{1}\left( x_{i} \right)}}.}} & (10)\end{matrix}$

Dividing Equation 10 by Equation 7 and generalizing the equation foreach x on G₁ yields

$\begin{matrix}\begin{matrix}{\frac{\Delta\; x}{{\sin\;{\beta_{1}\left( {x + {\Delta\; x}} \right)}} - {\sin\;{\beta_{1}(x)}}} = {\frac{v_{1}}{v_{2}}\left\lbrack {\frac{\left( {v_{1} - v_{2}} \right)\sin\;{\beta_{1}(x)}}{\cos^{3}{\beta_{1}(x)}{\delta\rho}} - \frac{1}{\cos^{3}{\beta_{1}(x)}}} \right\rbrack}} \\{= {\frac{a\;\sin\;{\beta_{1}(x)}}{\cos^{3}{\beta_{1}(x)}} - \frac{b}{\cos^{3}{\beta_{1}(x)}}}}\end{matrix} & (11)\end{matrix}$where the constants a and b are defined as a≡(ν₁−ν₂)ν₁/(ν₂δρ) andb≡ν₁/ν₂. Defining f(x)≡ sin β₁(x) yields

$\begin{matrix}{\frac{\Delta\; x}{{f\left( {x + {\Delta\; x}} \right)} - {f(x)}} = {\frac{{af}(x)}{\left( {1 - {f^{2}(x)}} \right)_{2}^{3}} - {\frac{b}{\left( {1 - {f^{2}(x)}} \right)_{2}^{3}}.}}} & (12)\end{matrix}$

For a system with a small change in the refractive index, it may beassumed that Δx<<1. Hence, the following approximation may be written:

$\begin{matrix}{\frac{\Delta\; x}{{f\left( {x + {\Delta\; x}} \right)} - {f(x)}} \approx {\frac{\mathbb{d}x}{\mathbb{d}f}.}} & (13)\end{matrix}$

Inserting Equation 12 into Equation 13 yields the following differentialequation:

$\begin{matrix}{\frac{\mathbb{d}x}{\mathbb{d}f} = {\frac{{af}(x)}{\left( {1 - {f^{2}(x)}} \right)_{2}^{3}} - {\frac{b}{\left( {1 - {f^{2}(x)}} \right)^{\frac{3}{2}}}.}}} & (14)\end{matrix}$

The solution of this equation is

$\begin{matrix}{{x = {\frac{a - {bf}}{\sqrt{1 - f^{2}}} + c_{0}}},} & (15)\end{matrix}$where c₀ is found from the boundary condition (f(x=0)=0) to bec ₀ =−a   (16)

Thus, the solution of Equation 15 is

$\begin{matrix}\begin{matrix}{{\sin\;{\beta_{1}(x)}} = {f(x)}} \\{= {\frac{{ab} + \sqrt{({ab})^{2} - {\left( {a^{2} - \left( {x + a} \right)^{2}} \right)\left( {b^{2} + \left( {x + a} \right)^{2}} \right)}}}{b^{2} + \left( {x + a} \right)^{2}}.}}\end{matrix} & (17)\end{matrix}$

Inserting Equation 18 into Equation 1 shows the grating function φ₁(x)of grating G₁ to be

$\begin{matrix}{{\phi_{1}(x)} = {{\frac{2\pi}{\lambda_{1}}\left\lbrack {{v_{1}\frac{{ab} + \sqrt{\begin{matrix}{({ab})^{2} - \left( {a^{2} - \left( {x + a} \right)^{2}} \right)} \\\left( {b^{2} + \left( {x + a} \right)^{2}} \right)\end{matrix}}}{b^{2} + \left( {x + a} \right)^{2}}} + {\sin\;\beta_{0}}} \right\rbrack}.}} & (18)\end{matrix}$

The grating function φ₂(ξ) of the second grating G₂ can be calculated bya similar procedure to that described above with regard to Equations1-18. The result of these calculations is

$\begin{matrix}{{{\phi_{2}(\xi)} = {\frac{2\pi}{\lambda_{1}}\left\lbrack \frac{{a^{\prime}b^{\prime}} + \sqrt{\begin{matrix}{\left( {a^{\prime}b^{\prime}} \right)^{2} - \left( {a^{\prime\; 2} - \left( {x + a^{\prime}} \right)^{2}} \right)} \\\left( {b^{\prime\; 2} + \left( {x + a^{\prime}} \right)^{2}} \right)\end{matrix}}}{b^{\prime\; 2} + \left( {x + a^{\prime}} \right)^{2}} \right\rbrack}},} & (19)\end{matrix}$where the constants a′ and b′ are now defined as a′≡(ν₁−ν₂)ν₁/(ν₂δρ) andb≡ν₁/ν₂−1.

It is important to note that the solution given in Equations 18 and 19is not the most accurate analytical one, but rather, is an approximatesolution, illustrating the capability of finding an easy and fastanalytical solution for the aforementioned iterative process. However,for most cases, as will be described in further detail below, thissolution is accurate enough.

In other cases, in which the system has a high numerical aperture, adiffraction-limited performance is required; hence, as the accuracy ofthe solution is crucial, a more accurate analytical solution must befound. This can be done by using a more accurate value for Δx inEquation 10, or by finding higher coefficients in the power series ofEquation 13. Furthermore, after calculating the values of φ₁ and φ₂ onthe discrete points x₁,x₂,x₃ . . . x_(i) and ξ₁,ξ₂,ξ₃ . . . ξ_(i)respectively, as described earlier, a numerical or semi-numerical methodcan be employed to find the required accurate solution.

In addition, the solution, even the most accurate one, is computed fortwo discrete values of the refractive index, ν₁ and ν₂, but it can beassumed that for the dynamic range of ν={ν₁Δν/2, ν₂+Δν/2}, whereΔν≡ν₂−ν₁, a change of the refractive index to ν yields a deviation ofthe output wave to the directionsin ρ^(ν)=sin ρ₁ +δρ·g(ν),   (20)where g(ν) is a monotonic function having the values of g(ν)=0,1 forν=ν₁,ν₂, respectively. Since g(ν)=x is a continuous and monotonicfunction, the inverse function g⁻¹(x)=ν can also be found.

The angular steering of the output wave can be translated into linearscanning. As shown in FIG. 4, there are provided two gratings, G₁ andG₂, parallel to each other and located at the surfaces of alight-transmissive substrate 4, where the refractive index of thesubstrate can be dynamically controlled. A focusing lens 28 is providedat a certain distance from grating G₂, forming a focus at the imagingplane 30. The angular steering of the plane wave is converted by thefocusing lens 28 into linear scanning of a point. Each plane wave,corresponding to a different refractive index, is focused by thefocusing lens 28 onto the image plane 30, where the foci of the variousplane waves are laterally displaced along a straight line.

The beam steering discussed above is performed only in the x axis.However, a two-dimensional scanner can easily be fabricated by combiningtwo different parallel substrates, whereby the scanning direction ofeach substrate is normal to that of the other.

Another problem is that since the relation between the coordinates ofthe two gratings is x=ξ+tan β(ξ), the lateral dimension of the secondgrating is much smaller than that of the first grating in the x-axis,whereby the dimensions remain the same along the y-axis. That is, theoutput beam has a non-symmetrical form, whereby the lateral dimensionalong the y-axis is much larger than that along the x-axis. Apparently,this problem does not exist when the scanning system is composed of twosubstrates, as described above. But even for a single substrateconfiguration, this problem can be solved by contracting the widerdimension, using optical means such as a folding prism.

Thus far, the exploitation of the double grating configuration embeddedon a light-transmissive substrate, mainly for scanning purposes, hasbeen described. However, the same configuration could be used also forother applications, including wavelength-division multiplexing, opticalswitching, light intensity attenuation, light amplitude-modulation andmany others.

Considering now the above-described system of Equations 1-19, in thespecial case where β₀=0 and the refractive index ν₁ is fixed but thewavelength is changed from λ₁ to λ, the direction of the output ray foreach point x on the first grating G₁ is

$\begin{matrix}{{v_{1}\sin\;{\beta_{1}^{\lambda}(x)}} = {\frac{\lambda_{2}}{2\pi}{{\phi_{1}(x)}.}}} & (21)\end{matrix}$

Inserting Equation 1 into Equation 21 yields

$\begin{matrix}{{v_{1}\sin\;{\beta_{1}^{\lambda}(x)}} = {\frac{\lambda}{\lambda_{1}}v_{1}\sin\;{{\beta_{1}(x)}.}}} & (22)\end{matrix}$

Hence,

$\begin{matrix}{{\sin\;{\beta_{1}^{\lambda}(x)}} = {\frac{\lambda}{\lambda_{1}}\sin\;{{\beta_{1}(x)}.}}} & (23)\end{matrix}$

However, in the former case, where the wavelength λ=λ₁ is fixed and thevariable is the refractive index ν, the direction of the output ray foreach point x on the first grating G₁ is

$\begin{matrix}{{\sin\;{\beta_{1}^{v}(x)}} = {\frac{v_{1}}{v}\sin\;{{\beta_{1}(x)}.}}} & (24)\end{matrix}$

Hence, the output direction for each point x on the grating G₁ is equalin both cases, that issin β₁ ^(ν)(x)=sin β₁ ^(λ)(x),   (25)in a condition that

$\begin{matrix}{\lambda = {\frac{v_{1}}{v}{\lambda_{1}.}}} & (26)\end{matrix}$

Consequently, each ray with a direction sin β₁ ^(ν) impinges on thesecond grating G₂ at a point ξ where the grating function is

$\begin{matrix}{{\phi_{2}(\xi)} = {\frac{2\pi}{\lambda_{1}}{\left( {{v\;\sin\;{\beta_{1}(x)}} + {\sin\;\rho^{v}}} \right).}}} & (27)\end{matrix}$

The output direction for each point ξ on G₂ is now

$\begin{matrix}{{\sin\;{\rho^{\lambda}(\xi)}} = {{\frac{\lambda}{2\pi}{\phi_{2}(\xi)}} - {v_{1}\sin\;{{\beta_{1}^{\lambda}(x)}.}}}} & (28)\end{matrix}$

Inserting Equations 25 and 27 into Equation 28 yields

$\begin{matrix}{{\sin\;{\rho^{\lambda}(\xi)}} = {{\frac{\lambda}{\lambda_{1}}v\;\sin\;{\beta_{1}(x)}} + {\frac{\lambda}{\lambda_{1}}\sin\;\rho^{v}} - {\frac{\lambda}{\lambda_{1}}v\;\sin\;{{\beta_{1}(x)}.}}}} & (29)\end{matrix}$

Inserting Equations 20 and 26 into Equation 29 yields

$\begin{matrix}\begin{matrix}{{\sin\;{\rho^{\lambda}(\xi)}} = {\frac{\lambda}{\lambda_{1}}\sin\;\rho^{v}}} \\{= {\frac{\lambda}{\lambda_{1}}\left( {{\sin\;\rho_{1}} + {{\delta\rho} \cdot {g(v)}}} \right)}} \\{= {\frac{\lambda}{\lambda_{1}}{\left( {{\sin\;\rho_{1}} + {{\delta\rho} \cdot {g\left( \frac{v_{1}\lambda_{1}}{\lambda} \right)}}} \right).}}}\end{matrix} & (30)\end{matrix}$

Regarding now the more general case in which both the wavelength and therefractive index are variables, the direction of the output ray for eachpoint x on the first grating G₁ is

$\begin{matrix}{{\sin\;{\beta_{1}^{v,\lambda}(x)}} = {\frac{v_{1}}{v}\frac{\lambda}{\lambda_{1}}\sin\;{{\beta_{1}(x)}.}}} & (31)\end{matrix}$

This case is equivalent to changing only the refractive index to ν^(o),wherein

$\begin{matrix}{v^{o} = {\frac{v\;\lambda_{1}}{\lambda}.}} & (32)\end{matrix}$

Consequently, the output direction for each point ξ on G₂ is

$\begin{matrix}{{\sin\;{\rho^{v,\lambda}(\xi)}} = {\frac{\lambda}{\lambda_{1}}\sin\;{\rho^{v^{o}}.}}} & (33)\end{matrix}$

Inserting Equation 20, where ν^(o)=ν, into Equation 33 yields

$\begin{matrix}{{\sin\;{\rho^{v,\lambda}(\xi)}} = {\frac{\lambda}{\lambda_{1}}{\left( {{\sin\;\rho_{1}} + {{\delta\rho} \cdot {g\left( v^{o} \right)}}} \right).}}} & (34)\end{matrix}$

Inserting Equation 32 into Equation 34 yields

$\begin{matrix}{{\sin\;{\rho^{v,\lambda}(\xi)}} = {\frac{\lambda}{\lambda_{1}}{\left( {{\sin\;\rho_{1}} + {\delta\;{\rho \cdot {g\left( \frac{v\;\lambda_{1}}{\lambda} \right)}}}} \right).}}} & (35)\end{matrix}$

Seemingly, sin ρ^(ν,λ)(ξ)=sin ρ^(ν,λ) is a constant over the entire areaof the grating G₂. As a consequence, the output beam is a plane wave.Therefore, the present invention can also be used as awavelength-division multiplexing/demultiplexing device.

FIG. 5 illustrates the utilization of the double-grating configuration,constructed to provide a wavelength division demultiplexing systemincluding an optical device 32 linking a single source fiber 34 and aplurality of receiving fibers at receiving locations RL₁, RL₂. . .RL_(n). The source fiber 34 contains n different communication channels,CC₁. . . CC_(n), with the wavelengths, λ₁. . . λ_(n) respectively. Thefirst grating G₁ couples the corresponding incoming channels 101, 102into the light transmissive substrate 4, and the second grating G₂couples them 131, 132 out and diffracts them into different directions.Each channel CC_(i) is then focused by a focusing lens 36 onto itsreceiving fiber RL 35. The propagation direction of the waves can beinverted to provide a system which multiplexes a number of channels fromtheir separated source fibers onto one receiving fiber. Since the lighttransmissive substrate can be located very close to the fibers, and thelight waves are guided inside the substrate, the system can be compactand easy to use.

It is clear that the embodiment described above can also be used for WDMwith a substrate composed of a material with a constant refractiveindex, that is, when ν=ν₁=const. However, it is advantageous to usematerials in which the refractive index can be controlled. One of themain drawbacks of using a simple grating as a WDM device is that it isvery sensitive to the signal wavelength, which is usually stronglydependant on the temperature. In addition, since the diameter of thefiber core is smaller than 10 microns, the tolerances of the opticalsystem should be very tight, so as to prevent degradation of the opticalsignal due to even a slight change in the environmental conditions. Thenecessity for tightening the optical and the mechanical tolerances makesthe optical system very expensive and even impractical. In contrast to asimple grating, in the system according to the present invention,changes in environmental conditions can be compensated dynamically bychanging the refractive index. For example, since g⁻¹(x) is a continuousand monotonic function, it is possible to find a refractive index νwhich fulfills the relation

$\begin{matrix}{{v = {\frac{\lambda}{\lambda_{1}} \cdot {g^{- 1}\left( {\frac{\lambda_{1} - \lambda}{\lambda} \cdot \frac{\sin\;\rho_{1}}{\delta\rho}} \right)}}},{or}} & (36) \\{{{g\left( \frac{v\;\lambda_{1}}{\lambda} \right)} = \left( {\frac{\lambda_{1} - \lambda}{\lambda} \cdot \frac{\sin\;\rho_{1}}{\delta\rho}} \right)},} & (37)\end{matrix}$which yields

$\begin{matrix}{{\sin\;\rho_{1}} = {\frac{\lambda}{\lambda_{1}}{\left( {{\sin\;\rho_{1}} + {{\delta\rho} \cdot {g\left( \frac{v\;\lambda_{1}}{\lambda} \right)}}} \right).}}} & (38)\end{matrix}$

Substituting Equation 35 into Equation 38 yieldssin ρ₁=sin ρ^(ν,λ)  (39)That is, the output has the original direction as if the system has thevalues of ν₁ and λ₁. Consequently, compensation may be made for a changein the wavelength by changing the refractive index, and vice-versa. Inaddition, a taping element 35 attached to the receiving fibers andconnected to a control unit 37 of the refractive index, can produce aclosed-loop apparatus that will optimally control the performance of theoptical system.

Another potential application of the present invention is as an opticalswitch. In the embodiment illustrated in FIG. 5, each one of the opticalchannels CC₁ . . . CC_(n), having the wavelengths, λ₁ . . . λ_(n)respectively, can be routed into one of the output locations RL₁ . . .RL_(n). The routing of the channel CC_(i) to the output location RL₁ canbe done by setting a refractive index ν₁ ^(j) that will solve theequationsin ρ^(ν/λ)=sin ρ^(ν1,λj)  (40)

Inserting Equation 35 into Equation 37 yields the equation

$\begin{matrix}{{{\frac{\lambda_{i}}{\lambda_{1}}\left( {{\sin\;\rho_{1}} + {{\delta\rho} \cdot {g\left( \frac{v_{i}^{j}\lambda_{1}}{\lambda_{j}} \right)}}} \right)} = {\frac{\lambda_{j}}{\lambda_{1}}\left( {{\sin\;\rho_{1}} + {{\delta\rho} \cdot {g\left( \frac{v_{1}\lambda_{1}}{\lambda_{j}} \right)}}} \right)}},} & (41)\end{matrix}$which has the solution

$\begin{matrix}{{v_{i}^{j} = {\frac{\lambda}{\lambda_{1}} \cdot {g^{- 1}\left( {{\frac{\lambda_{1} - \lambda}{\lambda} \cdot \frac{\sin\;\rho_{1}}{\delta\rho}} + {\frac{\lambda_{j}}{\lambda_{i}}{g\left( \frac{v_{1}\lambda_{1}}{\lambda_{j}} \right)}}} \right)}}},} & (42)\end{matrix}$whereby ν₁ is the original refractive index designated to rout eachchannel CC_(k) to its respective default output location RL_(k).Apparently, this embodiment can be generalized to produce an opticalswitch between n optical channels CC₁ . . . CC_(n) and m possible outputlocations RL₁ . . . RL_(m), wherein n≠m.

Other potential applications of the present invention are in lightintensity attenuation, or light amplitude modulation. In manyfiber-optical applications, it is crucial to control the intensity ofthe light which is coupled into the fiber. The devices which arecurrently used for controlling coupled light intensity are mechanicalattenuators in the main, which are fairly expensive and suffer from aslow time response. In the embodiment of FIGS. 6 a and 6 b, the exactdirection of the optical wave W_(o) emerging from the substrate 4 can becontrolled by changing the refractive index of the substrate. Angularsteering of the wave is converted into linear shifting of the focusedwave by means of converging lens 36. Hence, the exact portion of thefocused wave to be coupled into the receiving fiber RL can be set, andthe device can be operated as an optical intensity attenuator (FIG. 6b). In addition, since the time response of the change in the refractiveindex is usually very fast, an intensity modulation of the coupled wavecan be performed with this device.

Another potential utilization of the present invention is as amonochromator. There are many applications in which it is desired toproduce a monochromatic beam out of an optical wave having a much widerspectrum, where the exact selected wavelength of the output should beset dynamically during the operation of the device. As described abovefor optical switching, if the detector receives light only from apre-determined direction, the desired wavelength can be set to emerge atthat direction by setting the appropriate refractive index. In addition,this device can also be used as a spectrometer by measuring theintensity of the output wave as a function of the wavelength.

The embodiments described above are merely examples illustrating theimplementation capabilities of the present invention. The invention canalso be utilized in many other potential applications, including, butnot limited to, angular drivers for laser range-finders, dynamic aimingsystems, laser beam steerer for CD-ROM readers and others, where dynamiccontrol of the direction of an optical wave is desired.

Another issue to be considered, and related principally to theutilization of the system for scanning purposes, is the dimensions ofthe light transmissive substrate. The relation between the apertures ofthe first grating G₁ and the second grating G₂ is given byx _(max)=ξ_(max)+tan β₁ ^(o)(ξ_(max)),   (43)where x_(max) and ξ_(max) are the apertures of the gratings G₁ and G₂,respectively. It can be seen that there are two contrary considerationsfor choosing ξ_(max). On the one hand, the size of the coupler apertureincreases with ξ_(max); hence, for a given aperture, increasing ξ_(max)decreases d, and thereby a more compact system is obtained. On the otherhand, the aberrations of the output wave also increase with β₁(ξ_(max)),which increases monotonically with ξ_(max); hence, by increasingξ_(max), the performance of the system is decreased. With theseconsiderations, one might think that an optimal value of ξ_(max) to bechosen for each different design, according to the desired system sizeand optical performance. Unfortunately, this optimal value of ξ_(max)usually does not exist. For example, a typical scanner aperture can beapproximately a few tens of mm, while the thickness of the substratecannot be more than 20-30 mm. On the other hand, the aberrations forξ_(max)>2·d are such that the optical performance is less than thediffraction limit. This limitation might be overcome by assembling anarray of n identical gratings, given by

$\begin{matrix}{{G_{j} = {{\sum\limits_{i = 1}^{n}{G_{j}^{i}\mspace{20mu} j}} = 1}},2,} & (44)\end{matrix}$where G^(k) _(j)=G^(i) _(j) for 1≦i,k≦n. For all 1≦i≦n the grating G₁^(i) is constructed in relation to the second grating G₂ ^(l).

Since the first grating G₁ is constructed of a large number of facets,the diffraction efficiencies of these facets should be properly set soas to achieve uniform illumination on the second grating G. In addition,special care should be taken to avoid spaces between the elements G₁^(i), in order to avoid loss of energy and to ensure an output wave withuniform intensity. To guarantee this, the coordinate x_(max) ^(i) of thefacet G₁ ^(i) should be identical to the coordinate x^(i+1)=0 of theadjacent facet G₁ ^(i+1). The total area of the grating G₁ is given byRL(G ₁)=n·x _(max) =n·(ξ_(max)+tan β₁(ξ_(max)))   (45)

For a given system, the parameter ξ_(max) is calculated according to thedesired performance and RL(G₁) is set according to the system aperture.Hence, it is possible to calculate the required number of facets n, fromEquation 45.

An alternative manner of reducing the thickness of the substrate inorder to achieve a more compact system is illustrated in FIG. 8. Thewave W₁, coupled inside the substrate by means of grating G₁, does notproceed directly to the second grating G₂, but is first reflected a fewtimes off the surfaces 38, 40 of the substrate 4. This reflectance canbe induced by a reflective coating on the surfaces of the substrate or,if the coupling angles are high enough, by total internal reflection. Inany case, the actual thickness of the substrate is d_(act)=d/(n+1),where n is the number of reflections of the substrate surfaces. Careshould be taken to avoid overlap between the coupled wave after thefirst reflection and the active area of the gratings. In the originalembodiment, which is described above, the light proceeds directly fromG₁ to G₂, and hence the gratings are located on opposite sides of thesubstrate. There are however, configurations with an odd number ofreflections from the substrate surfaces, in which the input and theimage waves could be located on the same side of the substrate.

An example of the device of FIG. 8 has the following parameters:δρ/(ν₂−ν₁)=5; ν₁=1.5; λ₁=1.5 μm; d=30 mm; ρ₁=20°; β₀=0   (46)

The maximum dispersion as a function of the refractive index for adouble grating designed according to Equations 18 and 19 can becalculated. The dispersion in the band is ν₁±Δν/2, where Δν≡0.02ν₁, andwhere x_(max), is d/3=10 mm, hence, ξ_(max) is set to be ˜1 mm. As aresult, the diffraction limit of the system is approximately 1.5milliradians. The beams are reflected twice off the surfaces of thesubstrate, hence, the actual thickness of substrate 4 is 10 mm.

FIG. 9 shows the angular dispersion as a function of the refractiveindex (normalized to ν₁). It is evident from this Figure that themaximal dispersion of the double grating is only 0.4 milliradians, whichis actually a diffraction limited performance for the system defined inEquation 46. It is even possible to increase the system aperture by afactor of three and still achieve a nearly diffraction limitedperformance of 0.5 milliradians.

FIG. 10 shows the output angle from the second grating as a function ofthe refractive index normalized to ν₁, for the system defined inEquation 46. The system has a scanning range of 100 milliradians (˜6°)for a refractive index with a dynamic range of Δν≡0.02ν₁.

FIG. 11 illustrates the utilization of the present invention as anoptical switch and/or as a monochromator. The figure depicts thewavelength, at an output angle of ρ=20°, as a function of the refractiveindex. By combining the results of FIGS. 10 and 11, it is clear to seethat a wavelength change of Δλ=15 nm yields an angular deviation of 0.5milliradians, which illustrates the performance of the device as a WDM.An even better spectral sensitivity can be achieved by selecting higherdeviation ratio (δρ/(ν₂−ν₁)) for the system.

FIGS. 12 a and 12 b show the grating period (in line-pairs/mm) of thegratings G₁ and G₂ as a function of x (FIG. 12 a) and ξ (FIG. 12 b),respectively. Both functions monotonically increase with ξ and x, andthe fabrication process for both gratings should be fairly simple.Furthermore, other gratings with constant grating periods, which can beeasily fabricated by conventional techniques such as holographicrecording or photo-lithography, may be added to the surfaces of thesubstrate. As a result, the maximal grating period values of thegratings G₁ and G₂ can be less than 250 line-pairs/mm.

A combination of the embodiments described above can be utilized tomaterialize an all-optical switching system, having as an input ncommunication channels and n different wavelengths λ₁ . . . λ_(n), andas an output m, receiving channels RC₁ . . . RC_(m) (not shown).

FIG. 13 illustrates a side view (a projection on the ε-ζ plan) of thefirst stage of the switching system. The n input channels 42 are coupledby a diffraction grating G₁ into the substrate 4, at a location wherethe substrate is constructed of a passive material. The pair of gratingsG₁, G₂ is used to perform the wavelength-division-demultiplexing, usingthe method described above with reference to FIG. 5. The second gratingtraps the output waves inside the substrate by total internalreflection.

FIG. 13 illustrates how a lateral separation between two differentchannels 44, 46, having the wavelengths λ_(i) and λ_(j), respectively,can be obtained. After a total lateral separation between the channelsis achieved, an array of gratings 48 ₁ to 48 _(n) is used to rotate thetrapped waves in such a way that the propagation direction of theseparated waves is along the η axis, which is normal to the figure plan.

FIG. 14 depicts a side view (a projection on the η-ζ plan) of the secondstage of the switching system. After rotation by the gratings 48, thetrapped waves impinge on an array of gratings 50, where the substrate atthis location is constructed of a dynamic material having a refractiveindex that can be controlled by an external voltage. Though it cannot beseen in the Figure, it is noted that the different channels arelaterally separated along the ξ axis, where each channel 44, 46 has itsrespective pair of gratings 50, 52 and a separate part of the substratewhere the refractive index of each part can be separately controlled.Each pair of gratings 50, 52 is used to control the output angles of thechannels 44, 46, using the method described above with reference to FIG.3. The gratings array 54 couples the trapped waves from the substrateonto a coupling optics 56, 58 to focus the waves into their respectivereceivers 60, 62.

For a multi-stage optical switching system, an optional array ofwavelength converters 64, 66 can be inserted into the optical pass ofthe waves, in order to convert the wavelength of the wave, which isrouted into a receiver RC_(k) into the wavelength λ_(k).

Another subject which should be addressed is the polarization of theincoming light. It is well known that it is simpler to design andfabricate diffracting gratings or elements using the electro-opticeffect for S-polarized light than it is for non-polarized or P-polarizedlight. There are cases where light sources like VCSELs (Vertical CatitySurface Emitting Lasers) are linearly polarized. However, there are manycases, especially those associated with fiber-optical communication,where the polarization of the incoming-beam is unknown.

This problem can be solved by utilizing a half-wavelength plate and apolarizing beam-splitter. As illustrated in FIG. 16, light beam 64,having undefined polarization, emerges from a light source 66 andimpinges on a polarizing beam-splitter 68. The part of the light beam 70having P-polarization continues in the same direction, while theS-polarized light beam 72 is reflected and impinges on a folding mirror74 which reflects the light again to its original direction. By using ahalf-wavelength plate 76, it is possible to rotate the polarization ofthe P-polarized light such that it is S-polarized in relation to thegrating plane. In such a way, the grating G₁ is illuminated withS-polarized light. This solution suffers, however, from two mainproblems: First, the cross-section of the incoming beam is notsymmetrical any more. That is, the lateral dimension of the beam in theξ-axis is twice as much as the lateral dimension in the η-axis. Inaddition, since the polarization of the incoming beam is unknown and canbe in any orientation, there is uncertainty regarding the energydistribution of the incoming beam along the ξ-axis. This undesginedenergy distribution presents a drawback for optical systems where adiffraction-limited performance is required.

FIG. 17 illustrates a modified version of FIG. 16, which solves thesetwo problems. After crossing the half-wavelength plate 76, the lightbeam 70 is rotated by the folding mirror 78 and then combined with thelight beam 72, using a 50% beam-splitter 80. The grating G₁ isilluminated now by an S-polarized, uniform and symmetrical light beam82. Half of the incoming energy 84 is lost during this process, butusually the energy of the incoming light beam is high enough to standthis loss.

Not only S-polarized incoming beams, but also any other linearlypolarized light beams, can be created with the system described above.

It will be evident to those skilled in the art that the invention is notlimited to the details of the foregoing illustrated embodiments and thatthe present invention may be embodied in other specific forms withoutdeparting from the spirit or essential attributes thereof. The presentembodiments are therefore to be considered in all respects asillustrative and not restrictive, the scope of the invention beingindicated by the appended claims rather than by the foregoingdescription, and all changes which come within the meaning and range ofequivalency of the claims are therefore intended to be embraced therein.

The invention claimed is:
 1. An optical device for the transmission of optical signals of different wavelengths from a single source to a plurality of receiving channels, comprising: at least two major surfaces, a first surface and a second surface delimiting a medium wherein the medium between the surfaces is composed of light-transparent material having a constant refractive index; a first grating (G₁) and a second different grating (G₂), stationary with respect to each other, each having an X-axis and a Y-axis, located on at least one of the major surfaces, at a constant distance from each other, wherein the two gratings are invariant in the Y-axis and solely dependent on the X-axis, each of the gratings, having a lateral dimension, with at least one sequence of a plurality of parallel lines, wherein the spacing between the lines of the first grating (G₁) gradually increases in a direction away from an edge of the grating up to a maximum distance, and wherein the spacing between the parallel lines of the second grating (G₂) increases in the same direction as the spacing of the first grating; a light source emitting light waves in a given spectral range, at least a first light wave (101) having a first wavelength and a second light wave (102) having a second wavelength, different from the wavelength of the first light wave (101) emanating from the light source, impinging on the first grating (G₁) as plane waves in the same direction, the first and second light waves (101, 102) diffracted by the first grating (G₁), and subsequently diffracted as output plane waves by the second grating (G₂), the first and second light waves (101, 102) impinging on the first grating at different portions along the X-axis being diffracted by the first grating (G₁) in different directions toward the second grating, and traverse the second grating (G₂), the diffracted first light wave (101) and second light wave (102) being diffracted respectively by the second grating in a first output direction and a second output direction, wherein the first and second output directions are different and directed to two receiving channels (35).
 2. The device according to claim 1, wherein the output light waves have a diffraction limited performance.
 3. (Presently Presented) The device according to claim 1, further comprising a plurality of optical transmission paths, one for each receiving channel, and a single optical transmission path for all the channels.
 4. The device according to claim 3, wherein each of the optical transmission paths is an optical fiber.
 5. The device according to claim 3, wherein the device multiplexes the optical signals transmitted from the a plurality of optical transmission paths to the a single optical transmission path.
 6. The device according to claim 3, wherein the device demultiplexes the optical signals transmitted from the single optical transmission path to the plurality of optical transmission paths.
 7. The device according to claim 1, further comprising a light transmissive substrate, wherein said two major surfaces are located on external surfaces of the substrate.
 8. The device according to claim 1, wherein the dimension of the second grating along the X-axis is smaller than the dimension of the first grating along the X-axis, while the dimensions of the gratings along the Y-axis are substantially the same.
 9. An optical device for transmission of optical signals to a plurality of receiving channels of different wavelengths, the device defining an angular dispersion and comprising at least two major surfaces, a first surface and a second surface delimiting a medium, wherein the medium between the surfaces is composed of a light-transparent material with a constant refractive index; a first grating (G₁) and a second different grating (G₂), stationary with respect to each other, each having an X-axis and a Y-axis, located on at least one of the major surfaces, at a constant distance from each other wherein the grating functions of the two gratings are invariant in the Y-axis and depend solely on the X-axis; each of the gratings, having a lateral dimension, includes at least one sequence of a plurality of parallel lines, wherein the spacing between the lines of the first grating (G₁). gradually increases in a direction away from an edge of the grating up to a maximum distance, and wherein the spacing between the parallel lines of the second grating (G₂) increases in the same direction as the spacing of the first grating; a light source emitting collimated light waves located in a given spectral range between a first wavelength and a second wavelength; at least two input collimated light waves, each having a specific wavelength in the given spectral range emanating from the light source, diffracted by the first grating (G₁), and subsequently traverse the second grating (G₂) and diffracted as output plane light waves by the second grating (G₂) in different output directions; the output direction of each light wave determined by the wavelength of the light beam waves; wherein the angular dispersion between the two different light beams waves diffracted by the second grating, is at least 5 milliradians over 1.5 nm.
 10. The device according to claim 9, wherein the maximal grating period values of the two gratings is less than 250 line-pairs/mm.
 11. The device according to claim 9, wherein the width of the spectral range is at least 2% of the central wavelength of the spectral range.
 12. The device according to claim 9, wherein the central wavelength of the spectral range is around 1.5 μm.
 13. An optical device for transmission of optical signals to a plurality of receiving channels of different wavelengths, the device comprising at least two major surfaces, a first surface and a second surface delimiting a medium, wherein the medium between the surfaces is composed of a light-transparent material with a constant refractive index; a first grating (G₁) and a second different grating (G₂), stationary with respect to each other, each having an X-axis and a Y-axis, located on at least one of the major surfaces, at a constant distance from each other, wherein the grating functions of the two gratings are invariant in the Y-axis and depend solely on the X-axis; each of the gratings, having a lateral dimension, and including at least one sequence of a plurality of parallel lines, wherein the spacing between the lines of the first grating (G₁) gradually increase in a direction away from an edge of the grating up to a maximum, and wherein the spacing between the parallel lines of the second grating (G₂) increase in the same direction as the spacing between the parallel lines of the first grating; a light source emitting collimated light waves located in a given spectral range between a first wavelength and a second wavelength; at least two collimated light waves (101, 102) each having a specific wavelength in the given spectral range emanating from the light source, diffracted by the first grating (G₁), the light waves impinging on the first grating (G₁) are directed towards the second grating (G₂), traverse the second grating and subsequently diffracted as output plane light waves (131, 132) by the second grating (G₂) in different output directions, the output direction of each light wave determined by the wavelength of the light waves; wherein the two light waves having the first and second wavelengths, respectively, are diffracted by the second grating (G₂) into first and second directions, respectively, the ratio of the difference between the first and second directions and the difference between the first and second wavelengths, is at least 5 milliradians over 1.5 nm.
 14. The device according to claim 13, wherein the maximal grating period values of the two gratings is less than 250 line-pairs/mm.
 15. An optical device for the transmission of optical signals from a plurality of emitting channels (35) of different wavelengths to a single receiver (34), the device comprising: at least two major surfaces, a first surface and a second surface delimiting a medium, wherein the medium between the surfaces is composed of light-transparent material having a constant refractive index; a first grating (G₂) and a second different grating (G₁), stationary with respect to each other, each grating having an X-axis and a Y-axis, located on at least one of the major surfaces, at a constant distance from each other, the two gratings being invariant in the Y-axis and solely dependent on the X-axis, each of the gratings, having a lateral dimension, with at least one sequence of a plurality of parallel lines, wherein the spacing between the lines of the first grating (G₂) gradually increases in a direction away from an edge of the grating up to a maximum distance between the lines, and wherein the spacing between the parallel lines of the second grating (G₁) increases in the same direction as the spacing of the first grating; at least a first and a second light source (35) each emitting light waves in a given spectral range between a first wavelength and a second wavelength; the light emanating from the first and second light sources impinging on the first grating (G₂) as plane waves from two different directions, being diffracted by the first grating (G₂) traverses the second grating (G₁), and subsequently diffracted as output plane waves by the second grating (G₁), the light waves impinging on the first grating (G₂)at different points along the X-axis being diffracted by the first grating (G₂) in different directions toward the second grating (G₁); all light waves diffracted by the second grating (G₁) being diffracted at an output direction which is the same for all the light waves, and are directed into a single receiving channel (34).
 16. An optical device for the transmission of optical signals of different wavelengths from a single source to a plurality of receiving channels, comprising: at least two major surfaces, a first surface and a second surface delimiting a medium, wherein the medium between the surfaces is composed of light-transparent material having a constant refractive index; a first grating (G₁) and a second different grating (G₂), stationary with respect to each other, each having an X-axis and a Y-axis, located on at least one of the major surfaces, at a constant distance from each other, wherein the two gratings are invariant in the Y-axis and solely dependent on the X-axis; each of the gratings, having a lateral dimension, with at least one sequence of a plurality of parallel lines, wherein the spacing between the lines of the first grating (G₁) gradually increases in a direction away from an edge of the grating up to a maximum distance, and the spacing between the parallel lines of the second grating (G₂) increases in the same direction as the spacing of the first grating (G₁); the first grating (G₁) having at least a first and a second point along the X-axis, and the second grating (G₂) having at least a first, a second and a third point along the X-axis; a light source (34) emitting light waves located in a given spectral range; at least a first light wave (101) having a first wavelength (λ₁) and a second light wave (102) having a second wavelength (λ₂) different from the wavelength of the first light wave (101), emanating from the light source (34), impinging on the first grating (G₁) as plane waves in the same direction, each of the light waves (101, 102) having at least a first ray (111) and second ray (112), the first and second light waves (101, 102) diffracted by the first grating (G₁), traverses the second grating (G₂), and subsequently diffracted as output plane waves (131, 132) by the second grating (G₂); the first (111, 121) and second (112, 122) rays of the first (101) and second (102) light waves impinge on the first grating (G₁) in the same direction at the first (X₁) and second (X₂) point, respectively; the first (111, 121) and second (112, 122) rays of the first (101) and second (102) light waves are diffracted by the first grating (G) in different directions; the first (111) and second (112) rays of the first (101) light wave impinge on the second grating (G₂) at the first (ξ₀) and second (ξ₁) points respectively; the first (121) and second (122) light rays of the second (102) light wave impinge on the second grating (G₂) at the second (ξ₁) and third (ξ₂) points, respectively; the rays of each the first light wave (101) and second light wave (102) being diffracted by the second grating (G₂) in a first (131) output direction and a second (132) output direction respectively; wherein the first (131) and second (132) output directions are different and directed to two receiving channels.
 17. An optical device for the transmission of optical signals of different wavelengths from a plurality of sources (35) to a single receiving channel (34), the device comprising: at least two major surfaces, a first surface and a second surface delimiting a medium, wherein the medium between the surfaces is composed of light-transparent material having a constant refractive index; a first grating (G₂) and a second different grating (G₁), stationary with respect to each other, each grating having an X-axis and a Y-axis, located on at least one of the major surfaces, at a constant distance from each other, wherein the two gratings are invariant in the Y-axis and solely dependent on the X-axis, each grating having a lateral dimension, with at least one sequence of a plurality of parallel lines, wherein the spacing between the lines of the first grating (G₂) gradually increases in a direction away from an edge of the grating up to a maximum distance between the lines, and wherein the spacing between the parallel lines of the second grating (G₁) increases in the same direction as the spacing of the first grating (G₂); the first grating (G₂) having at least a first, a second and a third point along the X-axis and the second grating (G₁) having at least a first and a second point along the X-axis; at least a first and a second light source (RL₁, RL₂) each light source emitting light waves in a given spectral range between a first wavelength and a second wavelength; the light waves emanating from the light sources (RL₁, RL₂,) impinging on the first grating (G₂) as plane waves from two different directions (131, 132), being diffracted by the first grating (G₂) traverses the second grating (G₁), and subsequently diffracted as output plane waves (101, 102) by the second grating (G₁); each of the impinging light waves having at least a first (112, 122) and a second (111, 121) ray; the first ray (112) and the second ray (111) of the first light wave from the first light source (RL₁) impinging on the first grating (G₂) in the same direction (131) at the first (ξ₂) and the second (ξ₁) points, respectively; the first ray (122) and the second ray (121) of the light wave from the second light source (RL₂) impinging on the first grating (G₂) in the same direction (132) at the second (ξ₁) and the third (ξ₀) points, respectively; the first (112, 122) and the second (111, 121) rays of the light waves are diffracted by the first grating (G₂) in different directions; the first (112, 122) and second (111, 121) rays impinge on the second grating(G₁) at the first (X₂) and second (X₁) points, respectively; the first and second light waves being diffracted by the second grating (G₁)in a direction which is the same direction and into a single receiving channel (34). 